Sequential fuel injection systems have heretofore been proposed using one electrically operated valve for each cylinder to allow flow of a controlled amount of fuel during each intake stroke with the amount of fuel being controlled by controlling the duration of electrical pulses applied through distributor means to allocate the application of the pulses to the injector valves in accordance with the firing order of the engine. Such systems have not been entirely satisfactory with respect to construction, operation and reliability. For example, timing means for controlling the duration of opening of the injector valve means have generally included multivibrator means responsive to one or more control signals the magnitude of which are controlled by variations in operating parameters such as speed, intake mainfold pressure, etc. The injector valve opening time is a function of the magnitude of the control signals, the function being non-linear and being subject to variations with variations in the characteristics of components in the multivibrator circuitry. As a result, complexities have been introduced with respect to the generation of the control signals and, in addition, very close tolerances must be maintained to obtain uniform and reliable performance.